1. Field of the Invention
This invention relates to novel xanthene compounds and to their use, e.g., as light-screening dyes in photographic products and processes.
2. Description of the Prior Art
It is well known that photographic film, and especially multicolor films, may and generally do vary from lot to lot, notwithstanding efforts to "repeat" previous films. Manufacturers of multicolor photographic films have developed a number of procedures to minimize the effects upon the final multicolor image of unavoidable variations in the manufacturing operations. These variations are reflected primarily in shifts in color balance as reflected in mismatching of the D log E curves of the individual red, green and blue exposures. Equipment used to coat multicolor films is highly precise but variations between intended coverage of silver halide and/or the dye image-forming materials do occur. Repeat batches of silver halide emulsions may, and usually do, vary in their photographic response. Individual layers may be dried to slightly different degrees. Films are stored for a period of time after coating to allow the films to "age", so that changes in sensitometry following coating have an opportunity to reach a plateau prior to sale. If the film is designed to be developed by a photofinisher or in a darkroom, processing of the exposed multicolor film is controlled within very narrow limits, typically within plus or minus a half degree of a prescribed temperature, in order to minimize sensitometric variations from film to film. Where the multicolor film is of the negative type, an opportunity to adjust the sensitometry occurs in printing the desired final positive image, during which operation the printing exposure may be appropriately color filtered.
The basic sources of sensitometric variations noted above exist also in multicolor diffusion transfer films, with the added complication that once the film is shipped, the sensitometric properties are essentially fixed. The opportunity for adjustment provided in darkroom processing, practically speaking, is unavailable for users of self-developing films. While professional and advanced amateur photographers may be skillful enough to utilize color correction filters to at least partially "rebalance" the color balance, ordinary users of the film would only be confused by such additional operations.
It is well known to use light-screening dyes in photographic elements. Such a dye may be incorporated as a filter dye in a light-sensitive emulsion layer(s) or in a layer coated over one or more light-sensitive emulsion layers or between two differently color-sensitized emulsion layers to modify the light record in the emulsion layer or to control the spectral composition of light falling on the underlying light-sensitive layer, or it may be incorporated as an anti-halation dye in a non-light-sensitive layer positioned on either side of a support carrying the light-sensitive layer(s).
The dyes employed for these purposes, in addition to having the requisite spectral absorption characteristics for their intented use, should be photochemically inert, that is, they should not have any adverse effect on the properties of the light-sensitive emulsion layer(s), and also, they should be capable of being decolorized or removed during photographic processing so as not to leave stain in the processed photographic element. In photographic processes where the dye is removed by being dissolved in a processing solution, it is usually preferred that the dye also decolorize in order to avoid contamination of the processing solution and to prevent staining from residual dye in the processed light-sensitive element.
Though various classes of dyes have been proposed for use in antihalation and color correction filter layers, the dyes heretofore employed have not been altogether satisfactory. Some of the dyes tend to reduce sensitivity, fog or exert other adverse effect on the light-sensitive material. However, the major drawback of previously employed dyes is their tendency to cause strain due to incomplete decolorization or reversal of some of the decolorized form to the original colored form. For example, some classes of dyes rely on the presence of a reagent, such as, a sulfite for "bleaching", i.e., decolorization and unless the dyes are removed from the light-sensitive material during or after processing, their color may reappear in time.
Among the classes of light-screening dyes used previously are the triarylmethane and xanthenes. For example, U.S. Pat. Nos. 1,879,537; 1,994,876; 2,350,090 and 3,005,711 disclose the use of fuchsone-type dyes in antihalation layers, and U.S. Pat. Nos. 3,406,069 and 3,615,548 are concerned with the metal chelates of fuchsone dyes as antihalation dyes. These and other types of triarylmethane dyes suffer from one or more of the drawbacks discussed above, and in particular, prior dyes of this type have been difficult to keep decolorized at the pH's normally encountered during processing subsequent to "bleaching" and in the final product. Xanthenes have been employed in antihalation layers that are removed during photographic processing. For example, U.S. Pat. Nos. 2,182,794; 2,203,767 and 2,203,768 disclose the use of rhodamine dyes in certain antihalation layers that are removed during processing in an acid bath or a plain water rinse bath depending upon the solubility characteristics of the particular layer.
U.S. Pat. Nos. 4,258,118 and 4,304,834 disclose certain 3,6-disubstituted xanthene compounds possessing a substituted phenyl moiety in the 9-position which moiety has the formula ##STR3## wherein X is ##STR4## and R' is alkyl and Y is an electron-withdrawing group. In these compounds, the X group undergoes an irreversible cleavage reaction in base to provide a moiety that adds to the central carbon atom to form a new ring-closed compound which is colorless. As disclosed and claimed in these patents, these compounds are useful as photographic light-screening dyes which offer advantages over prior light-screening dyes because of their ability to decolorize completely and irreversibly to a substantially inert product that remains colorless over a broad pH range.
Xanthenes of the structure ##STR5## wherein R is H or --COCH.sub.3 are disclosed in Beilstein's Handbuch der Organischen Chemie, Vol. 27, p. 534. These compounds are synthesized by condensing m-diethylaminophenol with saccharin at a temperature of 165.degree. C. to give the compound wherein R is H, which compound is then heated to boiling with acetic acid anhydride to yield the N-acylated derivative. As reported therein, solutions of the N-acetyl compound, unlike solutions of the N-unsubstituted compound (R.dbd.H), are not decolorized by boiling, and the acetyl group splits off only after prolonged boiling with alcoholic sodium hydroxide. Further to the N-unsubstituted compound, it has been found that this compound is colorless at an alkaline pH, faintly colored at neutrality and becomes more intensely colored as the pH is reduced.